Representative prior art patents showing motor control systems and circuits include U.S. Pat. Nos. 3,571,681; 3,518,516; and 3,411,058. Typical prior art systems have certain drawbacks relating in general to the speed control for the motor. For example, when the speed control is set initially at an intermediate or high speed setting, many times, due to cold bearings or an existing overload, the motor does not start. The initial reaction on the part of the operator is to set the speed indicator still higher. This, however, compounds the problem. Some systems overcome this problem inefficiently by supplying increased current to the motor windings. For applications requiring low power drain this is an obvious disadvantage. Other systems require that the motor be modified such as by adding additional windings or optical or magnetic sensing means to determine motor speed. This would of course, increase the cost of the overall system. In accordance with the present invention, it is thus an object to provide an improved speed control wherein the motor can be brought up to speed quite rapidly without excess power drain and without initial stalling or synchronous slippage. In the present invention the motor is controlled from a variable oscillator and the clock frequency of the oscillator is controlled in accordance with the current delivered to the motor windings. Thus, initially as the speed control is quickly increased, (too quickly to maintain synchronous speed), the winding current increases and this indication is fed back to the clock circuit or oscillator to effectively slow down the clock thus enabling the rotor to come up to speed more rapidly without any loss of synchronism. In effect, the system of this invention is a torque regulator with a desired maximum speed as permitted by the torque requirements.
Another disadvantage that is inherent in some control circuits or systems is that there is no braking action provided when the operator returns the speed control to its low or zero setting. When the motor has been operated at high speed and with a normal relatively light load, it takes 10-20 seconds for the motor to coast to a stop.
Accordingly, it is an object of the present invention to provide a motor control system wherein there is provided an electronic braking action when the speed control returns to its zero.
Another object of the present invention is to provide a motor control system wherein the motor can be of conventional design without a need for modifying the motor such as by adding extra windings or employing photo cell or magnetic detectors.